Protein crystal structure solution by fast incorporation of negatively and positively charged anomalous scatterers

The preparation of derivatives by the traditional methods of soaking is one of the most time-consuming steps in protein crystal structure solution by X-ray diffraction techniques. The `quick cryosoaking' procedure for derivatization with halides (monovalent anions) offers the possibility of significantly speeding up this process [Dauter et al. (2000), Acta Cryst. D56, 232-237]. In the present work, an extension of this technique is proposed and the use of two different classes of compounds (monovalent and polyvalent cations) that can be successfully utilized in the quick cryosoaking procedure for the derivatization and phasing of protein crystals is described. This approach has been tested on hen egg-white lysozyme and has been successfully used to solve the structure of a novel trypsin inhibitor. The possibility of using cations in the fast cryosoaking procedure gives additional flexibility in the process of derivatization and increases the chances of success in phase determination. This method can be applied to high-throughput crystallographic projects.